The Telegraph suggests a number of free attractions in the UK with a special focus on half term activities for children.
Many museums have free entry - and a number also have free or minimal cost activities for children and young people. Always worth checking out.
One common theme I note here is the price of car parking at many attractions. So it can be frugal to park some way away from sites, and take a one or two mile walk - or use public transport if it's available.